Icosahedral modified generalized balanced ternary and aperture 3 hexagon tree

ABSTRACT

A method for assigning path address-form location codes to objects represented using aperture 3 hexagon discrete global grid systems in both vector systems and bucket and raster systems in which hexagons in a first resolution are given a linear code and hexagons in subsequent finer resolutions have identifiers added to the linear code, the method iteratively applying the assigning step to further finer resolutions to a maximum resolution. In vector systems each hexagon has seven hexagons in a finer resolution and in raster and bucket systems each hexagon is assigned to be an open or closed generator class, an open generator creating a closed generator in a finer resolution, and a closed generator generating six open generator hexagons and a seventh closed generator hexagon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/038,484, filed Jan. 21, 2005 (now U.S. Pat. No. 7,876,967), whichclaims the benefit of U.S. Provisional Application No. 60/537,506, filedJan. 21, 2004, both of which are incorporated herein by reference.

FIELD

The present invention relates to a hierarchical location coding methodfor geospatial computing on icosahedral aperture 3 hexagon discreteglobal grid systems (DGGS).

BACKGROUND

Calculations on computer systems involving the locations of objectssituated on, or referenced to, the surface of the earth require alocation coding system, a particular computer representation ofgeospatial location. Depending on the application, a location code mayrepresent a zero-dimensional point, a one-dimensional line/curve, or atwo-dimensional region, or a set of these location forms. A locationcoding must define a quantization operator which maps such locations onthe earth's surface to location codes, which consist of a string of bitson modern digital computers.

As defined in (Sahr, K., D. White, and A. J. Kimerling. 2003. “GeodesicDiscrete Global Grid Systems”, Cartography and Geographic InformationScience. 30(2):121-134), the contents of which are incorporated hereinby reference, a discrete global grid (DGG) is a set of polygonal regionsthat partition the surface of the earth, where each region hasassociated with it a single point contained within it (usually theregion center point). Each region/point pair is designated a cell. Adiscrete global grid system (DGGS) is a multi-resolution set of discreteglobal grids.

DGGS have been developed using multi-resolution aperture 3 hexagon gridstiled onto an icosahedron. These DGGS include the ISEA3H DGGS as definedin (Sahr, K., D. White, and A. J. Kimerling. 2003. “Geodesic DiscreteGlobal Grid Systems”, Cartography and Geographic Information Science.30(2):121-134), the contents of which are incorporated herein byreference. The cells of a hexagon-based icosahedral DGGS always includeexactly twelve pentagonal cells at each resolution; these are located atthe twelve vertices of the icosahedron.

The cells of any DGGS can have at least three relationships to thegeospatial locations being represented:

1. Vector systems (Dutton, G. 1989. “The fallacy of coordinates”,Multiple representations: Scientific report for the specialist meeting.Santa Barbara, National Center for Geographic Information and Analysis).For each resolution in a DGGS, each point on the earth's surface ismapped to the DGG cell point associated with the DGG cell region inwhich it occurs. Line/Curves can be represented as an ordered vector ofcell point vertexes. A region can be represented using a line/curverepresentation of it's boundary.

2. Raster systems. The areal units associated with the cells of the DGGSform the pixels of a raster system. For each resolution in a DGGS, eachpoint on the earth's surface is mapped to the DGG cell region in whichit occurs. Line/Curves can be represented as an ordered vector of cellregions intersected by the curve. A region can be represented using aline/curve representation of it's boundary, or as the set of pixelscontaining, intersecting, or contained by the location beingrepresented.

3. Bucket systems. The areal regions associated with the DGGS cells arebuckets into which data objects are assigned based on their location.Depending on the application, a data object can either be assigned tothe finest resolution cell which entirely contains it, or to thecoarsest resolution cell which uniquely distinguishes it from all otherdata objects in the data set of interest.

In order to be useful on a computer system each cell in a DGGS hasassigned to it one or more unique location codes, each of which is anaddress consisting of a string of bits on the computing system. Theselocation codes generally take one of two forms:

1. Pyramid Address (as outlined in Burt, P J, 1980, “Tree and PyramidStructures for Coding Hexagonally Sampled Binary Images”, ComputerGraphics and Image Processing, 14:271-280). Each cell is assigned aunique location code within the DGG of corresponding resolution. Thissingle-resolution location code may be multi-dimensional or linear.Given a resolution K cell with single-resolution location code K-add, wecan designate the DGGS pyramid location code to be the pair (K, K-add).A multi-resolution DGGS address representation of a location can beconstructed by taking the series of single-resolution DGG addressesordered by increasing resolution.

Pyramid addresses are useful in applications that make use of singleresolution data sets. Quantization operators to/from pyramid addressesare known for grids based on triangles, squares, diamonds, and hexagons.

2. Path Address. The resolution K quantization of a geospatial locationinto a DGGS cell restricts the possible resolution K+1 quantizationcells to those whose regions overlap or are contained within theresolution K cell. We can construct a spatial hierarchy by designatingthat each resolution K cell has as children all K+1 cells whose regionsoverlap or are contained within it's region. A path address is one whichspecifies the path through a spatial hierarchy that corresponds to amulti-resolution location quantization. Path addresses are often linear,with each digit corresponding to a single resolution and specifying aparticular child cell of the parent cell specified by the addressprefix.

The reduction in redundant location information allows path addresses tobe smaller than pyramid addresses. And because the number of digits in alocation code corresponds to the maximum resolution, or precision, ofthe address, path addresses automatically encode their precision(Dutton, G. 1989. “The fallacy of coordinates”, Multiplerepresentations: Scientific report for the specialist meeting. SantaBarbara, National Center for Geographic Information and Analysis).

Path addresses are also used to enable hierarchical algorithms. Inparticular, the coarser cell represented by the prefix of a locationcode can be used as a coarse filter for the proximity operationscontainment, equality, intersection/overlap, adjacency, and metricdistance. These proximity operations are the primary forms of spatialqueries used in spatial databases, and such queries are thus renderedmore efficient by the use of path addresses. For example, take a systemwhere data object boundaries are assigned to the smallest containingbucket cell. A query may ask for all data objects whose locationsintersect a particular region. We can immediately discard fromconsideration all objects in bucket cells that do not intersect thesmallest containing bucket cell of the query region (Samet, H. 1989. TheDesign and Analysis of Spatial Data Structures. Menlo Park, Calif.:Addison-Wesley).

Path addresses are usually associated with spatial hierarchies whichform traditional trees, where each child has one and only one parent.Such hierarchies include those created by recursively subdividingsquare, triangle, or diamond cell polygons. However, the spatialhierarchies formed by aperture 3 hexagon grids do not form trees; eachcell can have up to three parents. For this reason aperture 3 hexagongrids have previously been addressed using pyramid addressing systems.

SUMMARY

The present invention overcomes the deficiencies of the prior art bydefining a method for assigning path address-form location codes toobjects represented using aperture 3 hexagon DGGS. A method calledmodified generalized balanced ternary (MGBT) is introduced thatspecifies the assignment of path address-form location codes for caseswhen a planar aperture 3 hexagon grid is used as a vector system. Amethod called the aperture 3 hexagon tree (A3HT) is introduced thatspecifies the assignment of path address-form location codes for caseswhen a planar aperture 3 hexagon grid is used as a raster or bucketsystem. The method and system of the present invention further specifyone possible efficient tiling of planar patches of these path addresshierarchies onto the icosahedron, resulting in the icosahedral MGBT(iMGBT) and icosahedral A3HT (iA3HT) respectively. This fully specifiespath address-form location codes for aperture 3 hexagon DGGS such as theISEA3H when used as a vector, raster, or bucket system.

The present system further specifies a transformation to and from theselocation codes and an existing pyramid code system. Since algorithmsexist for the quantization of common location forms (such aslatitude/longitude coordinates) to/from these pyramid codes this fullyspecifies the quantization of location forms to/from the iMGBT and iA3HTsystems.

In one broad aspect, therefore, the present invention relates to amethod for assigning, in a vector system, path address-form locationcodes to objects represented using aperture 3 hexagon discrete globalgrid system where each hexagon in a first resolution has seven hexagonsassociated therewith in a next finer resolution to the first resolution,said seven hexagons being centered on each of the vertices of the firstresolution hexagon and on the center of the first resolution hexagon,the method comprising the steps of specifying a linear code for a firstresolution hexagon, assigning an identifier to each of the sevenhexagons in the next finer resolution to the first resolution, anditeratively applying the assigning step to further finer resolutionsuntil a maximum resolution is achieved.

In another broad aspect, the present invention relates to a method forassigning, in a bucket or a raster system, path address-form locationcodes to objects represented using aperture 3 hexagon discrete globalgrid system, the method comprising the steps of assigning a hexagon in afirst resolution as an open generator type or a closed generator type,if the first resolution hexagon is an open generator type, creating asingle hexagon in a next finer resolution to the first resolution, saidsingle hexagon being a closed generator type hexagon and centered on thefirst resolution hexagon, if the first resolution hexagon is a closedgenerator type, creating seven hexagons in a next finer resolution tothe first resolution, the seven hexagons including a closed generatorhexagon centered on the center of the first resolution hexagon and theremaining six hexagons each being an open generator type and centered onthe vertices of the first resolution hexagon, specifying a linear codefor the first resolution hexagon, adding a first identifier to a hexagonwith the same center as the first resolution hexagon and of the nextfiner resolution to the first resolution hexagon, adding identifiers toany hexagons in the next finer resolution to the first resolutionhexagon being centered on the vertices of the first resolution hexagon,and iteratively applying the creating and adding steps to further finerresolutions of hexagons until a maximum resolution is achieved.

The foregoing and other objects, features, and advantages of theinvention will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood with reference to thedrawings in which:

FIG. 1A is a diagram showing a three-axis hexagon coordinate system in afirst alignment;

FIG. 1B is a diagram showing a three-axis hexagon coordinate system in asecond alignment;

FIG. 2 is a Class 1 two-dimensional integer coordinate system;

FIG. 3 is a representation of four resolutions of an aperture 3 hexagongrid system;

FIG. 4 is a diagram showing the icosahedron divided up into 10quadrilaterals;

FIG. 5 shows the icosahedron unfolded onto the plane with numbered 2dicoordinate systems;

FIG. 6 illustrates the relationship between a resolution K pointquantization and possible resolution K+1 quantizations of the samepoint;

FIG. 7A illustrates a class 1 resolution K address;

FIG. 7B illustrates the assignment of resolution K+1 address digits;

FIG. 8A illustrates a class 2 resolution K address;

FIG. 8B illustrates the assignment of resolution K+1 address digits;

FIG. 9 shows iMGBT tiling on an unfolded icosahedron;

FIG. 10 shows the hexagons generated by an aperture 3 hexagon tree opengenerator hexagon;

FIG. 11 shows the hexagons generated by an aperture 3 hexagon treeclosed generator hexagon;

FIG. 12 shows the generation of four resolutions of an aperture 3hexagon tree grid from a closed initial generator;

FIG. 13A shows a Class 1 closed parent;

FIG. 13B shows the aperture 3 hexagon tree addresses generated by aClass 1 closed parent;

FIG. 14A shows a Class 2 closed parent;

FIG. 14B shows the aperture 3 hexagon tree addresses generated by aClass 2 closed parent;

FIG. 15 shows the cells generated by an aperture 3 pentagon tree opengenerator pentagon unfolded onto the plane;

FIG. 16 shows the cells generated by an aperture 3 pentagon tree closedgenerator pentagon unfolded onto the plane;

FIG. 17 shows iA3HT tiling on an unfolded icosahedron;

FIGS. 18A-18G illustrate the first seven resolutions of the iA3HTgeneration pattern on the ISEA3H DGGS; and

FIG. 19 shows a quadrilateral superimposed on an imaginary A3HThierarchy.

DETAILED DESCRIPTION

Reference is now made to FIG. 1. Traditional planar cartesian coordinatesystems employ two coordinate axes that are perpendicular to each other.Hexagon systems, however, naturally form three axes that are at 120degree angles to each other. There are two natural orientations of thesethree axes relative to the traditional cartesian coordinate system.

The first orientation as illustrated in FIG. 1A can be designated asClass I, and the orientation illustrated in FIG. 1B can be designated asClass II.

Of the I, J and K axes, two of these axes are sufficient to uniquelyidentify each hexagon within a hexagon coordinate system. Quantizationto/from cartesian coordinates is known (van Roessel, J. 1988.“Conversion of Cartesian coordinates from and to generalized balancedternary addresses.” Photogrammetric Engineering and Remote Sensing.54(11):1565-1570). In addition, two-axes coordinate systems have beenused in the development of a number of algorithms for hexagon grids,including:

-   -   metric distance (Luczak, E., and A. Rosenfeld. 1976. “Distance        on a Hexagonal Grid.” IEEE Transactions on Computers.        C-25(5):532-533)    -   vector addition and subtraction (Snyder, W., H. Qi, and W.        Sander. 1999. “A Coordinate System for Hexagonal Pixels”, SPI E,        San Diego, February 1999)    -   neighbor identification (Snyder, W., H. Qi, and W. Sander. 1999.        “A Coordinate System for Hexagonal Pixels”, SPIE, San Diego,        February 1999)    -   adapted Bresenham's line and circle rasterization (Wuthrich, C.        A., and P. Stucki. 1991. “An algorithmic comparison between        square- and hexagonal-based grids.” CVGIP: Graphical Models and        Image Processing. 53(4):324-339)    -   edge detection (Abu-Bakar, S. and R. J. Green. 1996. “Detection        of edges based on hexagonal pixel formats.” Proceedings of ICSP.        pp. 1114-1117)    -   line-of-sight (Verbrugge, C. 1997. “Hexgrids.” Unpublished        paper. McGill University)    -   field-of-view (Verbrugge, C. 1997. “Hexgrids.” Unpublished        paper. McGill University)    -   image gradient determination (Snyder, W., H. Qi, and W.        Sander. 1999. “A Coordinate System for Hexagonal Pixels”, SPIE,        San Diego, February 1999)    -   variable conductance diffusion (Snyder, W., H. Qi, and W.        Sander. 1999. “A Coordinate System for Hexagonal Pixels”, SPIE,        San Diego, February 1999)

Any alternative location coding for a hexagon grid (such as the MGBT andA3HT systems discussed below) trivially implement these algorithmsprovided a mapping is defined between the alternative system and thetwo-axes coordinate system. Location codes in the alternative coding areconverted to two-axes codes, and the algorithm is performed on thetwo-axes codes. Any resulting location codes are converted back to thealternative location coding.

Reference is now made to FIG. 2. The I and J axes are chosen as acoordinate system basis because they are most useful in constructingpyramid addresses for icosahedron-based DGGSs. The resulting coordinatesystem is the two-dimensional integer (2di) coordinate system asillustrated in FIG. 2. According to the method and system of the presentinvention, a multi-resolution aperture 3 hexagon grid system may beformed as follows.

Reference is made to FIG. 3. A single-resolution Class 1 hexagon grid isformed and designated as a resolution K grid. To form the next finergrid (resolution K+1), a Class 2 hexagon grid is created consisting ofhexagons with exactly one-third the area of the resolution K hexagonsand with the resolution K+1 hexagons centered on the vertices andcenter-points of the resolution K hexagons.

To obtain finer resolution hexagons, the above process is repeated tothe desired number of resolutions, alternating between Class 1 and Class2 grids at each successive resolution, as seen in FIG. 3.

As one skilled in the art will appreciate, the series may be startedwith a Class 2 grid and created with the successive resolutions ofalternating classes.

The icosahedron can be tiled with 2di coordinate systems by pairing the20 triangular faces into 10 quadrilaterals as illustrated in FIG. 4.Each 2di coordinate system origin corresponds to one of the pentagonalvertex cells. Two pentagonal vertex cells are left-over and can betreated as single-cell 2di coordinate systems at every resolution. FIG.5 shows 2di coordinate systems on an icosahedron unfolded onto the planeand given one possible numbering. The pentagonal cells have been drawnas hexagons.

A unique pyramid address of the form {r, [q, (i,j)]} may be assigned toeach cell in the multi-resolution grid, where r is the resolution of thehexagon, q is the quadrilateral 2di coordinate system on which the celloccurs, and (i,j) is the 2di address of the hexagon on the quadrilateralq resolution r grid. Class 2 resolutions do not conveniently follow theClass 1 coordinate axes naturally defined by the quadrilateral edges. Wenote that for any Class 2 resolution K there are Class 1 resolution K+1cells centered on each resolution K cell. Thus without ambiguity we canassign to each resolution K cell the coordinates of the class 1resolution K+1 cell centered upon it. This icosahedral coordinate systemis designated the q2di (quadrilateral 2di) system (Sahr, K. 2002.DGGRID: User Documentation for Discrete Global Grid Software. 27 pp.)

Let (x, y) be a point location on the plane. Then a resolution Kaperture 3 hexagon grid quantization of this point restricts thepossible resolution K+1 quantization of the point to the sevenresolution K+1 hexagons that overlap the resolution K hexagon. This isillustrated in FIG. 6.

A common form for linear codes consists of a string of digits, whereeach digit specifies a choice among the possible hexagons at aparticular resolution. Given a linear code for a resolution K cell in aplanar aperture 3 hexagon grid, a multi-scale hierarchical coding schemecan be specified by assigning specific digits to each of the sevenpossible resolution K+1 cells, and then applying this scheme iterativelyuntil the desired maximum resolution is achieved.

An address can be assigned in a fashion inspired by the GeneralizedBalance Ternary (GBT) system for hexagon aggregation and thus we callthis system Modified GBT (MGBT). FIGS. 7 and 8 show the assignment ofresolution K+1 address digits based on class 1/class 2 resolution Kaddresses respectively.

Since at each resolution there are seven possible hexagons, it ispossible for each digit to be represented in three bits. Note that threebits can represent eight distinct digits. The decimal digits 0-6 havebeen assigned according to the above. The remaining eighth digit,decimal 7 or binary 111, can serve a number of useful functions. If theaddresses are variable length, then a 7-digit can be concatenated to anaddress to indicate address termination. If the addresses are fixedlength, a 7-digit can be used to indicate that the remaining highresolution digits are all center digits (i.e., zero), and therefore thatadditional resolution will not add information to the location.

MGBT addressing can be applied to the earth's surface by tiling anaperture 3 hexagon DGGS such as the ISEA3H with MGBT tiles. Tilescentered on the twelve icosahedral vertices form pentagons and thusrequire special tiling units. These can be constructed by deletingone-sixth of the sub-hierarchy generated in the hexagon case. These areindicated using the procedure outlined for MGBT tiles except thatpentagonal tiles have a single sub-digit sequence deleted. That is, forpentagonal tiles with address A, all sub-tiles are indexed as per thecorresponding MGBT indexing except that sub-tiles with sub-indexes ofthe form AZd are not generated, where Z is a string of 0 or more zeroesand d is the sub-digit sequence (1, 2, 3, 4, 5, or 6) chosen fordeletion. All hierarchical descendants of such tiles are likewise notindexed. We use MGBT-d to indicate an MGBT tile with sub-digit sequenced deleted (e.g., MGBT-2 would indicate an MGBT tile with sub-digitsequence 2 chosen for deletion).

Given hexagonal and pentagonal MGBT tiles, we may tile the icosahedronto create the icosahedral MGBT (iMGBT). In order to fully specify ageospatial coding system based on the iMGBT, a fixed orientation must bespecified for each tile. One approach to achieving this is to unfold theicosahedron onto the plane and then specify that each tile be orientedconsistently with this planer tiling. FIG. 9 shows one such orientation.As one skilled in the art would appreciate, the specification of asingle triangular face on the unfolded icosahedron in FIG. 9unambiguously specifies the remaining relative positions of the other 19icosahedral faces. Each base tile is labeled with a tile number thatdesignates the base geospatial code of that tile.

Based on the above, an iMGBT location code on an icosahedral aperture 3hexagon DGGS can be indexed in at least four easily inter-convertibleforms.

-   -   1. Character string code form. The location code consists of a        string of digits beginning with the two-digit base tile code        (00, 01, 02, . . . 31) followed by the digit string        corresponding to the appropriate address of each finer        resolution within the tile.    -   2. Integer code form. The character string code form may be        interpreted and stored as a single integer value.    -   3. Modified integer code form. When displaying integer values        leading zeroes are usually removed. This will result in        differences in the number of digits between codes on base tiles        00-09 and those of the same resolution but on other base tiles.        This can be remedied by adding the value of 40 to each of the        base tile values so that they are numbered 40, 41, . . . 71.        Note that since digits 4-7 are not used as leading digits in the        integer code form, these address can be unambiguously        distinguished as being modified integer codes.    -   4. Packed code form. Under this form, the base tile codes are        stored as five-digit binary numbers. Sub-codes within each tile        are stored as a packed series of three-bit binary digits (as        described above) and appended to the base tile number to fully        specify a geospatial code. Under this scheme, codes up to        resolution 10 can be stored in 32 bits of contiguous storage,        and codes up to resolution 20 can be stored in 64 bits of        contiguous storage.

An iMGBT code for a point location can be truncated such that any prefixof the code yields a valid quantification of the point location at acoarser grid resolution. This allows prefixes of the code to be used asa coarse filter for the proximity operations equality, adjacency, andmetric distance.

While useful as a vector location system, iMGBT effectively addressessub-regions of cells and thus assigns multiple codes to many of thecells in an aperture 3 hexagon DGGS. This makes it unsuitable for rasteror bucket systems, where it is useful to have a specific unique code foreach cell. For applications where a unique code is required we introducethe planar aperture 3 hexagon tree (A3HT).

Each hexagon in an A3HT is assigned one of two generator hexagon types:open (type A), or closed (type B). As seen in FIG. 10, an open generatorat resolution K generates a single resolution K+1 hexagon that is aclosed generator centered on itself.

Reference is now made to FIG. 11. A closed generator hexagon atresolution K also generates a single resolution K+1 closed generatorhexagon at its center. The closed generator in addition generates sixresolution K+1 open generator hexagons, one centered at each of its sixvertices.

An A3HT of arbitrary resolution can be created by beginning with asingle open or closed hexagon and then recursively applying the abovegenerator rules until the desired resolution is reached. FIG. 12 showsthe first four resolutions of an A3HT generated by a resolution K closedgenerator hexagon.

Hexagons in an A3HT may be addressed by using a selection of MGBT codes.In all cases (Class 1 or Class 2, open or closed generator), the addressof centroid children are formed by concatenating a zero digit with theparent hexagon address. The addresses of vertex children of closedgenerators are formed by concatenating one of the digits 1-6 with theparent hexagon address; the particular digit chosen is based on theparent alignment class and the direction of the child relative to itsgenerated parent as illustrated for closed Class 1 and Class 2 parenthexagons in FIGS. 13 and 14 respectively.

A3HT addressing can be applied to the earth's surface by tiling anaperture 3 hexagon DGGS such as the ISEA3H with A3HT tiles. Tilescentered on the twelve icosahedral vertices form pentagons and thusrequire special tiling units.

Aperture 3 pentagon tree (A3PT) tiling units are generated similarly toA3HT tiles except that one-sixth of the sub-hierarchy is deleted. Eachpentagon in an A3PT is assigned one of two generator pentagon types:open (type A) or closed (type B). As seen in FIG. 15, an open A3PTgenerator at resolution K generates a single resolution K+1 pentagonthat is a closed generator centered on itself.

As illustrated in FIG. 16, a closed A3PT generator at resolution K alsogenerates a single resolution K+1 closed A3PT generator pentagon at itscenter, but in addition it generates five resolution K+1 open A3HT.generator hexagons, one centered at each of its five vertices.

An A3PT of arbitrary resolution can be created by beginning with asingle open or closed pentagon and then recursively applying the abovegenerator rules until the desired resolution is reached.

A3PT tiles can be indexed using the procedure outlined for A3HT tilesexcept that pentagon tiles have a single sub-digit sequence deleted.That is, for an A3PT tile with an address A, all sub-tiles are generatedas per the corresponding A3HT indexing except that sub-tiles withindexes of the form AZd are not generated, where Z is a string of 0 ormore zeroes and d is the sub-digit sequence (1, 2, 3, 4, 5, or 6) chosenfor deletion. All hierarchical descendants of such tiles are likewisenot generated.

A3PT-d is used to indicate an A3PT tile with the sub-digit sequence ddeleted. For example, A3PT-2 would indicate an A3PT tile with thesub-digit sequence 2 chosen for deletion.

Given A3HT and A3PT tiles, we may tile the icosahedron to create theicosahedral A3HT (iA3HT). In order to fully specify a geospatiallocation coding system based on the iA3HT, a fixed orientation must bespecified for each tile. One approach to achieving this is to unfold theicosahedron into the plane and then specify that each tile be orientedconsistently with this planer tiling. FIG. 17 shows one suchorientation. As one skilled in the art would appreciate, thespecification of a single triangular face on the unfolded icosahedron inFIG. 17 unambiguously specifies the remaining relative positions of theother 19 icosahedral faces. Each base tile is labeled with a tile numberthat designates the base geospatial code of that tile.

Reference is now made to FIG. 18. FIG. 18 illustrates the first sevenresolutions of an A3HT generation pattern on the ISEA3H DGGS.

Based on the above, an iA3HT location code on an icosahedral aperture 3hexagon DGGS can be indexed using any of the four code forms previouslydefined for the iMGBT: character string code form, integer code form,modified integer code form, or packed code form.

We note that since each open A3HT or A3PT generator generates only asingle center hexagon, every 1-6 digit in an iA3HT location code must befollowed by a 0 digit. A first order compression of the codes can beachieved by eliminating these redundant 0 digits.

The development of algorithms that can operate on iA3HT codes isongoing. Until such algorithms are available, the existing body ofalgorithms for 2di pyramid addresses can be exploited by specifyingalgorithms to convert between the two systems. First we definealgorithms for conversions between planar A3HT and 2di pyramid systems.We then extend these algorithms to DGGS by defining the conversionbetween q2di coordinates and iA3HT codes.

The definitions that follow assume that the root (resolution 0 hexagon)of all A3HT indices are Class 1 type A generators. Algorithm definitionsfor initial generators that are Class 2 and/or type B follow triviallyfrom the algorithms given below. The existence of the following known ortrivial algorithms is also assumed:

digit:A3HT×integer->integer. Returns a specific resolution digit from anA3HT code.

distance:2di×2di->integer. Return the metric distance between two 2diaddresses.

generateNextLevel:A3HT->{A3HT}. Returns the child indices of an A3HTcode.

isClass1:A3HT->boolean. Returns whether or not an A3HT code is Class 1.

isTypeA:A3HT->boolean. Returns whether or not an A3HT code is type A.

resolution:A3HT->integer. Returns the resolution of an A3HT code.

resolve:A3HT×integer->A3HT. Extend an A3HT code to a specifiedresolution by padding it with trailing 0's.

Conversion from A3HT to 2di Pyramid Coordinates

The algorithm a3ht2Coord2di gives a 2di address corresponding to a givenA3HT code. Algorithms used within this definition are given followingthe definition.

Algorithm a3ht2Coord2di:A3HT->2di

Convert A3HT code ndx to a 2di address coord of the same resolution asndx.

BEGIN a3ht2Coord2di coord <− (0, 0) sameClass <− true resOffset <− 0 fori = resolution(ndx) to 0 step by −1 if sameClass coord <− coord +sameClassDownN(digit(ndx, i), resOffset) else if (isClass1(ndx)) coord<− coord + class2downOne(digit(ndx, i)) * 3.0(resOffset−1)/2 else coord<− coord + class1downOne(digit(ndx, i)) * 3.0(resOffset−1)/2 end if-elseend if-else sameClass <− not sameClass resOffset <− resOffset + 1 endfor return coord END a3ht2Coord2diAlgorithm sameClassDownN:integer×integer->2di

Given an A3HT digit calculate the corresponding 2di coord on the grid nresolutions finer than the grid on which the A3HT digit is defined.Assumes both grids are of the same class.

BEGIN sameClassDownN if digit = 0 coord <− (0, 0) else if digit = 1coord <− (0, 1) else if digit = 2 coord <− (1, 0) else if digit = 3coord <− (1, 1) else if digit = 4 coord <− (−1, −1) else if digit = 5coord <− (−1, 0) else if digit = 6 coord <− (0, −1) end if-else coord <−coord * 3.On/2 return coord END sameClassDownNAlgorithm class2downOne:integer->2di

Given an A3HT digit defined on a Class 2 grid calculate thecorresponding 2di coord on the Class 1 grid one resolution finer.

BEGIN class2downOne if digit = 0 coord <− (0, 0) else if digit = 1 coord<− (1, 2) else if digit = 2 coord <− (1, −1) else if digit = 3 coord <−(2, 1) else if digit = 4 coord <− (−2, −1) else if digit = 5 coord <−(−1, 1) else if digit = 6 coord <−(−1, −2) end if-else return coord ENDclass2downOneAlgorithm class1downOne:integer->2di

Given an A3HT digit defined on a Class 1 grid calculate thecorresponding 2di coord on the Class 2 grid one resolution finer.

BEGIN class1downOne if digit = 0 coord <− (0, 0) else if digit = 1 coord<− (−1, 1) else if digit = 2 coord <− (2, 1) else if digit = 3 coord <−(1, 2) else if digit = 4 coord <−(−1, −2) else if digit = 5 coord <−(−2, −1) else if digit = 6 coord <− (1, −1) end if-else return coord ENDclass1downOneConversion from 2di Pyramid Coordinates to A3HT

The algorithm coord2di2A3HT gives an A3HT code corresponding to a 2dipyramid address. Algorithms used within this definition are givenfollowing the definition.

Algorithm coord2di2A3HT:2di×integer->A3HT

Convert 2di address coord of resolution finalRes to an A3HT code ndx.

BEGIN coord2di2A3HT if finalRes = 0 return 0 end if if finalRes = 1return 00 end if oldSet <− null set newSet <− generateNextLevel(00) forres = 2 to finalRes − 1 step by 1 oldSet <− newSet newSet <− null setfor each ndx in oldSet if confirmedDescendent(ndx, coord, res) newSet <−generateNextLevel(ndx) break for else if possibleDescendent(ndx, coord,res) newSet <− newSet + generateNextLevel(ndx) end else-if end for endfor for each ndx in newSet if coord = a3ht2Coord2di(ndx) return ndx endif end for END coord2di2A3HTAlgorithm confirmedDescendent:A3HT×2di×integer->boolean

Determine whether or not the 2di address coord of resolution res isdefinitely a descendent of the A3HT code ndx. Note that the vectorradius used in this algorithm is empirically derived.

BEGIN confirmedDescendent radius <− { 0, 0, 1, 1, 3, 4, 9, 13, 27, 40,81, 121, 243, 364, 729, 1093, 2187, 3280, 6561, 9841, 19683, 29524,59049, 88573, 177147, 265720, 531441, 797161, 1594323, 2391484} ifisTypeA(ndx) resDiff <− res − resolution(ndx) else resDiff <− res −resolution(ndx) + 1 end if-else if distance(coord,a3ht2Coord2di(resolve(ndx, res))) > radius(resDiff) return false elsereturn true end if-else END confirmedDescendentAlgorithm possibleDescendent:A3HT×2di×integer->boolean

Determine whether or not the 2di address coord of resolution res ispossibly a descendent of the A3HT code ndx. Note that the vector radiusused in this algorithm is empirically derived.

BEGIN possibleDescendent radius <− { 0, 0, 1, 2, 4, 8, 13, 26, 40, 80,121, 242, 364, 728, 1093, 2186, 3280, 6560, 9841, 19682, 29524, 59048,88573, 177146, 265720, 531440, 797161, 1594322, 2391484, 4782968} ifisTypeA(ndx) resDiff <− res − resolution(ndx) else resDiff <− res −resolution(ndx) + 1 end if-else if distance(coord,a3ht2Coord2di(resolve(ndx, res))) > radius(resDiff) return false elsereturn true end if-else END possibleDescendent

In addition to the algorithms for conversion from 2di coordinates toA3HT codes, we require an algorithm for rotating A3HT codes. To rotatean A3HT cell 60 degrees counter-clockwise about the center of the baseA3HT generator cell perform the following substitution on all digits inthe cell code:

original digit rotated digit 0 0 1 5 2 3 3 1 4 6 5 4 6 2

For example, the hexA0506 (where A is the base cell code) rotated 60degrees counter-clockwise would have a new address of A0402.

This algorithm can be efficiently implemented by creating separatetables for 60, 120, 180, 240, and 300 degrees, clockwise andcounter-clockwise, through multiple application of the table above.Further efficiency can be gained by generating tables for multiple digitsub-sequences, rather than for single digits alone.

We now define an algorithm for conversion from q2di coordinates to iA3HTcodes. Note that the iA3HT base cells pictured in FIG. 17 correspond toresolution 1 of the q2di system. Thus q2di resolution 0 has nocorresponding representation under iA3HT. Also note that the Class 1axes of the q2di system illustrated in FIG. 5 are rotated 60 degreesclockwise relative to the iA3HT Class 1 axes shown in FIG. 17.

Let A^n be the resolution n iA3HT code consisting of the base code Afollowed by n−1 zeroes. For example, the iA3HT coordinate 0600000 couldbe written 06^6. Note that all q2di coordinates of the form {r, [q, (0,0)]} correspond to vertices of the icosahedron and map directly to iA3HTcoordinates A^r, where A is the iA3HT base code corresponding to theorigin of the quadrilateral q. Table 1 lists the corresponding iA3HTbase code for each quadrilateral. Note that since quadrilaterals 0 and11 consist of only a single origin cell, this table fully defines thetransformation from these q2di quadrilaterals to iA3HT.

TABLE 1 Quadrilateral q vs. iA3HT base index A q 0 1 2 3 4 5 6 7 8 9 1011 A 00 10 06 07 08 09 21 22 23 24 25 31

Temporarily ignoring the complications introduced by the pentagon cells,it is useful to note that we can view each quadrilateral as lying on aclosed A3HT generator two resolutions coarser than the base iA3HT tiles.FIG. 19 illustrates this relationship. The iA3HT Class 1 axes are shownin dashed lines, while the quadrilateral Class 1 axes are solid gray.Assuming the base closed A3HT generator has address X, FIG. 19 gives thecorresponding codes for all A3HT cells coincident with thequadrilateral.

The iA3HT resolution 0 image in FIG. 19 provides the key to transformingfrom q2di to iA3HT codes. The basic steps for transforming a non-originClass 1 q2di coordinate {r, [q, (i, j)]} to iA3HT are as follows:

1. Perform a transformation (as defined above) from 2di pyramidcoordinate [r+1, (i, j)] to the corresponding A3HT code in the systemdefined by the Class 2 closed generator X (as shown in FIG. 19). Assumethe 2di axes are aligned with the A3HT Class 1 axes (dashed axes in FIG.19).

2. Rotate the A3HT address 60 degrees clock-wise to compensate for theq2di axes offset.

3. Replace the first 3 digits of the address with the correspondingiA3HT base cell code found in Table 2.

TABLE 2 iA3HT Base Cell Look-up Q2DI A3HT iA3HT Cell 00/31 InterruptionInterruption Quad q Prefix Base Cell Sub-Rotation Prefix Sub-Rotation 1X00 10 — — — X02 11 — — — X03 01 — — — X20 06 — 061 60° CW X30 00  0° —— X60 21 — — — 2 X00 06 — — — X02 12 — — — X03 02 — — — X20 07 — 071 60°CW X30 00  60° CCW — — X60 22 — — — 3 X00 07 — — — X02 13 — — — X03 03 —— — X20 08 — 081 60° CW X30 00 120° CCW — — X60 23 — — — 4 X00 08 — — —X02 14 — — — X03 04 — — — X20 09 — 091 60° CW X30 00 180° CW 001 60° CCWX60 24 — — — 5 X00 09 — — — X02 15 — — — X03 05 — — — X20 10 — 101 60°CW X30 00  60° CW — — X60 25 — — — 6 X00 21 — 216 60° CW X02 26 — — —X03 16 — — — X20 22 — — — X30 06 — — — X60 31  60° CW — — 7 X00 22 — 22660° CW X02 27 — — — X03 17 — — — X20 23 — — — X30 07 — — — X60 31 180°CW 316 60° CCW 8 X00 23 — 236 60° CW X02 28 — — — X03 18 — — — X20 24 —— — X30 08 — — — X60 31 120° CW — — 9 X00 24 — 246 60° CW X02 29 — — —X03 19 — — — X20 25 — — — X30 09 — — — X60 31  60° CCW — — 10 X00 25 —256 60° CW X02 30 — — — X03 20 — — — X20 21 — — — X30 10 — — — X60 31 0° — —

For most cells the resulting address will be the correct iA3HT address.In two cases additional adjustments are necessary. These adjustmentsrequire that the cell in question be rotated in 60 degree incrementsabout its center-point. This transformation can be accomplished bytranslating the cell center to the origin, performing the rotation, andthen translating the cell center back to its original position. In thecase of A3HT cells the same effect can be achieved by applying therotation algorithm to all digits except the base cell code. We call thisa sub-rotation.

The first case requiring adjustment is that iA3HT base cells 0 and 31require additional sub-rotations to account for their unique orientationrelative to the quadrilaterals. These rotations are indicated in thefourth column of Table 2.

In the second case we must check for cases where the generated addresslies on a deleted sub-sequence of an A3PT tile. These cases occur whenthe cell has a prefix as indicated in the fifth column of Table 2. Inthese cases we must perform the sub-rotation indicated in the sixthcolumn so that the appropriate existing portion of the pentagon replacesthe interruption.

Finally, note that, as previously discussed, Class 2 q2di cells areaddressed using the codes of the next finer resolution Class 1q2di grid.Given a Class 2 q2di address {r, [q, (i, j)]} first apply the aboveprocedure for the address {r+1, [q, (i, j)]}. Since this address is, bydefinition, the Class 1 center cell of the desired Class 2 cell, theresulting iA3HT code will be the address of the desired Class 2 cellwith an extra final digit of zero. Dropping this final zero digit fromthe code will give the correct code corresponding to the desired Class 2resolution cell.

The above therefore describes, in a bucket, raster or vector system,path address-form location codes and a method of assigning these codesto objects represented using an aperture 3 hexagon discrete global gridsystem. As would be appreciated by those skilled in the art, the abovedescription is not meant to be limiting, and the only limitations on thepresent invention are those set forth in the following claims.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. I thereforeclaim as my invention all that comes within the scope and spirit ofthese claims.

1. A method of assigning a location identifier associated with an object, the method comprising: identifying a first hexagon that includes a location of the object, the first hexagon being associated with a first resolution and being either an open type hexagon or a closed type hexagon; assigning, using a computer, a first location identifier associated with the object based on the first hexagon; and storing the first location identifier in a storage device.
 2. The method of claim 1, further comprising; identifying a second hexagon that includes a location of the object, the second hexagon being a child of a parent hexagon associated with the first resolution, the second hexagon being either an open type hexagon or a closed type hexagon; and assigning a second location identifier associated with the object based on the parent hexagon and the second hexagon.
 3. The method of claim 2, wherein the first hexagon is the parent hexagon.
 4. The method of claim 2, wherein the first hexagon is not the parent hexagon.
 5. The method of claim 2, wherein the parent hexagon is an open type hexagon and has a single child hexagon, the single child hexagon being centered on a center of the parent hexagon.
 6. The method of claim 2, wherein the parent hexagon is a closed type hexagon and has seven child hexagons, the seven child hexagons including one child hexagon centered on the center of the parent hexagon and six child hexagons centered on the vertices of the parent hexagon.
 7. The method of claim 6, wherein the seven child hexagons collectively cover a parent hexagon area.
 8. The method of claim 2, wherein the second location identifier comprises a first element representing the parent hexagon and a second element representing the second hexagon, the second element indicating a location of the second hexagon relative to the parent hexagon.
 9. The method of claim 1, wherein the first hexagon is associated with a surface of an icosahedron.
 10. One or more non-transitory computer-readable storage media comprising computer-executable instructions for performing a method of assigning a location identifer associated with an object, the method comprising: identifying a first hexagon that includes a location of the object, the first hexagon being associated with a first resolution and being either an open type hexagon or a closed type hexagon; assigning, using a computer, a first location identifer associated with the object based on the first hexagon; and storing the first location identifier in a storage device.
 11. A computer configured to assign a location identifier associated with an object, the computer comprising: an input portion configured to receive an indication of a first hexagon, the first hexagon including a location of the object, the first hexagon being associated with a first resolution and being either an open type hexagon or a closed type hexagon; and an assignment portion configured to assign a first location identifier associated with the object based on the first hexagon.
 12. The computer of claim 11, wherein: the input portion is further configured to receive an indication of a second hexagon that includes a location of the object, the second hexagon being a child of a parent hexagon associated with the first resolution, the second hexagon being either an open type hexagon or a closed type hexagon; and the assignment portion is further configured to assign a second location identifier associated with the object based on the parent hexagon and the second hexagon.
 13. The computer of claim 12, wherein the second location identifier comprises a path address-form location code having a first element representing the parent hexagon and a second element representing the second hexagon, the second element indicating a location of the second hexagon relative to the parent hexagon.
 14. The computer of claim 11, wherein the first hexagon defines an area on an icosahedral surface.
 15. The computer of claim 14, wherein the icosahedral surface represents a surface of a substantially spherical body and the first location identifier is associated with a location on the surface of the body.
 16. A method of converting between location code systems, the method comprising: identifying a first location code for an object, the first location code being of a system selected from a two-dimensional integer coordinate system and an aperture 3 hexagon tree system; determining, using a computer, a second location code for the object based on the first location code, the second location code being of a two-dimensional integer coordinate system if the first location code is of an aperture 3 hexagon tree system, and the second location code being of an aperture 3 hexagon tree system if the first location code is of a two-dimensional integer coordinate system; and storing the second location code in a storage device.
 17. The method of claim 16, wherein the two-dimensional integer coordinate system is a quadrilateral two-dimensional integer coordinate system and the aperture 3 hexagon tree system is an icosahedral aperture 3 hexagon tree system.
 18. The method of claim 17, wherein a location code of the quadrilateral two-dimensional integer coordinate system comprises an indication of a resolution of the location code, an indication a quadrilateral that includes the object, and an indication a two-dimensional position of the object within the quadrilateral.
 19. The method of claim 17, further comprising: transforming a two-dimensional integer pyramid code to an initial aperture 3 hexagon tree code; obtaining a rotated aperture 3 hexagon tree code associated with a 60 degree clockwise rotation of a coordinate system associated with the initial aperture 3 hexagon tree code; and replacing a portion of the rotated aperture 3 hexagon tree code with a corresponding iA3HT base cell code.
 20. One or more non-transitory computer-readable storage media comprising computer-executable instructions for performing a method of converting between location code systems, the method comprising: identifying a first location code for an object, the first location code being of a system selected from a two-dimensional integer coordinate system and an aperture 3 hexagon tree system; determining, using a computer, a second location code for the object based on the first location code, the second location code being of a two-dimensional integer coordinate system if the first location code is of an aperture 3 hexagon tree system, and the second location code being of an aperture 3 hexagon tree system if the first location code is of a two-dimensional integer coordinate system; and storing the second location code in a storage device.
 21. The one or more non-transitory computer-readable storage media of claim 20, wherein the two-dimensional integer coordinate system is a quadrilateral two-dimensional integer coordinate system and the aperture 3 hexagon tree system is an icosahedral aperture 3 hexagon tree system.
 22. The one or more non-transitory computer-readable storage media of claim 21, wherein a location code of the quadrilateral two-dimensional integer coordinate system comprises an indication of a resolution of the location code, an indication a quadrilateral that includes the object, and an indication a two-dimensional position of the object within the quadrilateral.
 23. The one or more non-transitory computer-readable storage media of claim 21, wherein the method further comprises: transforming a two-dimensional integer pyramid code to an initial aperture 3 hexagon tree code; obtaining a rotated aperture 3 hexagon tree code associated with a 60 degree clockwise rotation of a coordinate system associated with the initial aperture 3 hexagon tree code; and replacing a portion of the rotated aperture 3 hexagon tree code with a corresponding iA3HT base cell code. 